For all they (Aldean and security) knew, the shots came from the crowd. It only lasted 10 minutes, so by the time people knew it wasn't coming from the crowd (from their perspective back stage), it probably had already ended. Hell, there's a video floating around where conspiracy nutjobs were worried about someone pulling a gun in the crowd. You can kind of see a guy in that video wearing a security or cop uniform with what appears to be a gun drawn, but even if it was a gun drawn, I can't fault the guy for being ready.
Basically all I'm trying to say is that I think it's hard to criticize anything. I wanted to fault them for putting the lights up too, initially, but it's not clear at this time what would've been best.